Increasing solar cell efficiency is one of the most important goals for the solar cell research community, with the goal of enabling the lowest possible cost per watt for a given solar cell. A majority of current solar cells are fabricated on p-type Czochralski (Cz) silicon wafers with an n+ emitter layer. A problem with these conventional p-type silicon solar cells is that they suffer from light induced degradation and lower tolerance to metal impurities, which limits their efficiency to lower than 20%. As such, the particular goal of the solar cell research community is to develop high efficiency silicon solar cells having target efficiencies higher than 20%.
One approach currently being considered by the solar cell research community for achieving the >20% target efficiency is to use n-type silicon wafers with a p+ emitter layer in place of the currently used p-type wafers. N-type silicon wafers are known to avoid light induced degradation and have a higher tolerance to metal impurities than p-type silicon wafers, and therefore are believed to provide a solution for producing higher efficiency solar cells.
A problem currently faced by the solar cell research community in fabricating solar cells on n-type silicon wafers is finding a suitable dielectric material that can both passivate the p+ emitter layer, and can also be appropriately patterned to provide electrical connection to selected regions of the p+ emitter layer. The passivation (dielectric) layers on current solar cells fabricated on p-type wafers typically include silicon-nitride (SiNx) that is deposited using plasma-enhanced chemical vapor deposition (PECVD). Unfortunately, while PECVD deposited SiNx dielectric layer can effectively passivate the n+ emitter layer on p-type silicon wafers and has been used in mass production for many years, PECVD deposited SiNx cannot be used as a passivation layer on the p+ emitter layer of n-type silicon wafers because SiNx can only provide a positive fixed charge density, and thus can only passivate the surface where the minority carrier is holes (positive charger, that is n+ surface).
Recent study has proved that aluminum oxide (Al2O3) that is deposited using an atomic layer deposition (ALD) process is one of the most promising materials to passivate the p+ emitter of an n-type solar cell, and as such there has been a significant amount of research on ALD deposited Al2O3 in the past a couple of years. That is, in order to compete with the very low labor costs available to Asian companies, Western solar cell manufacturers are forced to adopt high efficiency solar cell production processes, and the formation of passivation layers using ALD deposited Al2O3 is believed to be one of the critical technologies for allowing high efficiency silicon solar cell production. Moreover, ALD deposited Al2O3 dielectric film can also effectively passivate the n+ emitter layer as well, making it very promising for high efficiency interdigitated back contact (IBC) solar cells where both p+ and n+ emitter layers are on the same side (backside) and need to be passivated simultaneously.
The current problem facing the solar cell research community in utilizing ALD deposited Al2O3 dielectric films is that, unlike PECVD deposited SiNx dielectric layers, silver paste can not fire through the ALD deposited Al2O3 layer to make the metal contact with the underlying p+ emitter layer. Currently, most of the cells made in laboratory use photolithography method to make contact openings through the Al2O3 layer, but this approach cannot be used in mass production due to the intrinsic high cost associated with the use of photolithography. Thus, how to achieve low-cost metallization through Al2O3 passivation layer is one of the bottlenecks for the mass production of high efficiency solar cells passivated with Al2O3.
What is needed is a low cost method for facilitating the mass production of high efficiency n-type silicon solar cells with the p+ emitter layers that addresses the problems set forth above. What is also needed are mass produced, high efficiency n-type silicon solar cells with p+ emitter layers that are manufactured using the method.